The invention relates in general to resonator structures of radio communication apparatus, especially bulk acoustic wave filter structures. In particular, the invention is directed to balanced radio frequency filter structures.
The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units. The development leads to increasing requirements on the miniaturization of the components and structures used in the mobile communication means. This development concerns radio frequency (RF) filter structures as well, which despite the increasing miniaturization should be able to withstand considerable power levels, have very steep passband edges, and low losses.
The RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. Surface acoustic wave (SAW) resonators typically have a structure similar to that shown in FIG. 1. Surface acoustic resonators utilize surface acoustic vibration modes of a solid surface, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. A SAW resonator typically comprises a piezoelectric layer 100, and two electrodes 122, 124. Various resonator structures such as filters are produced with SAW resonators. A SAW resonator has the advantage of having a very small size, but unfortunately cannot withstand high power levels.
It is known to construct thin film bulk acoustic wave resonators on semiconductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled xe2x80x9cAcoustic Bulk Wave Composite Resonatorsxe2x80x9d, Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layers of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled xe2x80x9cAn Air-Gap Type Piezoelectric Composite Thin Film Resonatorxe2x80x9d, I5 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, a bulk acoustic wave resonator having a bridge structure is disclosed.
FIG. 2 shows one example of a bulk acoustic wave resonator having a bridge structure. The structure comprises a membrane 130 deposited on a substrate 200. The resonator further comprises a bottom electrode 110 on the membrane, a piezoelectric layer 100, and a top electrode 120. A gap 210 is created between the membrane and the substrate by etching away some of the substrate from the top side. The gap serves as an acoustic isolator, essentially isolating the vibrating resonator structure from the substrate.
Bulk acoustic wave resonators are not yet in widespread use, partly due to the reason that feasible ways of combining such resonators with other circuitry have not been presented. However, BAW resonators have some advantages as compared to SAW resonators. For example, BAW structures have a better tolerance of high power levels.
In the following, certain types of BAW resonators are described first.
Bulk acoustic wave resonators are typically fabricated on silicon (Si), gallium arsenide (GaAs), glass, or ceramic substrates. One further ceramic substrate type used is alumina. The BAW devices are typically manufactured using various thin film manufacturing techniques, such as for example sputtering, vacuum evaporation or chemical vapor deposition. BAW devices utilize a piezoelectric thin film layer for generating the acoustic bulk waves. The resonance frequencies of typical BAW devices range from 0.5 GHz to 5 GHz, depending on the size and materials of the device. BAW resonators exhibit the typical series and parallel resonances of crystal resonators. The resonance frequencies are determined mainly by the material of the resonator and the dimensions of the layers of the resonator.
A typical BAW resonator consists of three basic elements:
an acoustically active piezoelectric layer,
electrodes on opposite sides of the piezoelectric layer, and
acoustical isolation from the substrate.
The piezoelectric layer may be for example, ZnO, AlN, ZnS or any other piezoelectric material that can be fabricated as a thin film. As a further example, also ferroelectric ceramics can be used as the piezoelectric material. For example, PbTiO3 and Pb(ZrxTi1-x)O3 and other members of the so called lead lanthanum zirconate titanate family can be used.
Preferably, the material used to form the electrode layers is an electrically conductive material having a high acoustic impedance. The electrodes may be comprised of for example any suitable metal, such as tungsten (W), aluminum (Al), copper (Cu), molybdenum (Mo), nickel (Ni), titanium (Ti), niobium (Nb), silver (Ag), gold (Au), and tantalum (Ta). The substrate is typicall composed of for example Si, SiO2, GaAs, glass, or ceramic materials.
The acoustical isolation can be produced with for example the following techniques:
with a substrate via-hole,
with a micromechanical bridge structure, or
with an acoustic mirror structure.
In the via-hole and bridge structures, the acoustically reflecting surfaces are the air interfaces below and above the devices. The bridge structure is typically manufactured using a sacrificial layer, which is etched away to produce a free-standing structure. Use of a sacrificial layer makes it possible to use a wide variety of substrate materials, since the substrate does not need to be modified very much, as in the via-hole structure. A bridge structure can also be produced using an etch pit structure, in which case a pit has to be etched in the substrate or the material layer below the BAW resonator in order to produce the free standing bridge structure.
FIG. 3 illustrates one example of various ways of producing a bridge structure. Before the deposition of other layers of the BAW structure, a sacrificial layer 135 is deposited and patterned first. The rest of the BAW structure is deposited and patterned partly on top of the sacrificial layer 135. After the rest of the BAW structure is completed, the sacrificial layer 135 is etched away. FIG. 3 shows also the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The sacrificial layer can be realized using for example a metal or a polymer as the material.
In the via-hole structure, the resonator is acoustically isolated from the substrate by etching away the substrate from under a major portion of the BAW resonator structure. FIG. 4 illustrates a via-hole structure of a BAW resonator. FIG. 4 shows the substrate 200, a membrane layer 130, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. A via-hole 211 has been etched through the whole substrate. Due to the etching required, via-hole structures are commonly realized only with Si or GaAs substrates.
A further way to isolate a BAW resonator from the substrate is by using an acoustical mirror structure. The acoustical mirror structure performs the isolation by reflecting the acoustic wave back to the resonator structure. An acoustical mirror typically comprise several layers having a thickness of one quarter wavelength at the center frequency, alternating layers having differing acoustical impedances. The number of layers in an acoustic mirror is typically an odd integer, typically ranging from three to nine. The ratio of acoustic impedance of two consecutive layers should be large in order to present as low acoustic impedance as possible to the BAW resonator, instead of the relatively high impedance of the substrate material. In the case of a piezoelectric layer that is one quarter of the wavelength thick, the mirror layers are chosen so that as high acoustic impedance as possible is presented to the resonator. This is disclosed in U.S. Pat. No. 5,373,268. The material of the high impedance layers can be for example gold (Au), molybdenum (Mo), or tungsten (W), and the material of the low impedance layers can be for example silicon (Si), polysilicon (poly-Si), silicon dioxide (SiO2), aluminum (Al), or a polymer. Since in structures utilizing an acoustical mirror structure, the resonator is isolated from the substrate and the substrate is not modified very much, a wide variety of materials can be used as a substrate. The polymer layer may be comprised of any polymer material having a low loss characteristic and a low acoustic impedance. Preferably, the polymer material is such that it can withstand temperatures of at least 350xc2x0 C., since relatively high temperatures may be achieved during deposition of other layers of the acoustical mirror structure and other structures. The polymer layer may be comprised of, by example, polyimide, cyclotene, a carbon-based material, a silicon-based material or any other suitable material.
FIG. 5 shows an example of a BAW resonator on top of an acoustical mirror structure. FIG. 5 shows the substrate 200, the bottom electrode 110, the piezoelectric layer 100, and the top electrode 120. The acoustical mirror structure 150 comprises in this example three layers 150a, 150b. Two of the layers 150a are formed of a first material, and the third layer 150b in between the two layers is formed from a second material. The first and second materials have different acoustical impedances as described previously. The order of the materials can be varied. For example, the material with a high acoustical impedance can be in the middle and the material with a low acoustical impedance on both sides of the middle material, or vice versa. The bottom electrode may also be used as one layer of the acoustical mirror.
FIG. 6 shows a further example of a BAW resonator structure. The BAW resonator illustrated in FIG. 6 is a stacked resonator structure having two piezoelectric layers 100. In addition to the bottom 110 and top 120 electrodes, a stacked structure requires a middle electrode 115, which is connected to ground potential.
FIG. 6 further shows the membrane layer 130, the substrate 200 and the etch pit 210 isolating the structure from the substrate.
FIG. 7a shows a schematic diagram of a lattice filter structure constructed using bulk acoustic wave resonators. A lattice filter consisting of BAW resonators is usually designed so that two of the four resonators i.e. resonators A have higher resonance frequencies than the resonators B. Typically the series resonance of resonators A is at or close to the parallel resonance frequency of the resonators B, which is the center frequency of the filter. The difference in the resonance frequencies can be achieved for example in the same way as typically done in BAW filters having a ladder structure, namely by increasing the thickness of one of the layers of the B resonators or depositing an additional layer on top of the B resonators. The additional layer, sometimes called the tuning layer, can be either a metal or a dielectric layer. An example of the layout of such a lattice structure is shown in FIG. 7b. Typically, the size of the resonators is determined by the desired impedance level of the filter. The impedance level is determined mainly by the inherent shunt capacitance C0 of the resonators, i.e. the capacitance between the top and bottom electrodes.
As explained in Finnish patent application FI982824, two of the four resonators of the lattice structure may have a larger area than the other two. The resonators having a large area can be either the resonators marked with B in FIG. 7a or the resonators marked with A in FIG. 7a. When the ratio of the areas is suitably chosen, the frequency response of such a balanced filter has a very steep attenuation slope outside the passband. This is in many applications a very desirable property of a band pass filter.
In this specification and especially in the accompanied claims, the term area of a resonator refers to the cross sectional area of the resonator, the cross section being taken in a plane substantially parallel to the substrate surface and the area being covered by both the top and the bottom electrodes. Although in the example of FIG. 7a the piezoelectric layers of the four resonators are separate, the piezoelectric layers of the resonators may form a single continuous layer. In this case the area of the resonator is defined substantially by the overlapping area of the top and the bottom electrodes at the location, where the overlapping occurs.
Balanced filters are typically band pass filters. The frequency response of a balanced filter is substantially symmetric around a certain frequency which is called the center frequency. Balanced filters are typically designed so that their impedance level at the center frequency gives optimal matching to the surrounding circuitry or, in other words, to the terminating impedances. Generally the resonators marked with A and B in. FIG. 7a have equal impedance levels, but it is also possible to design the balanced filter structure so that the geometric mean of the impedances of the resonators A and B is equal to the terminating impedances.
In general it is very important that a balanced filter has a certain specified frequency response and a certain specified impedance. The frequency response defines, for example, the radio frequencies where the filter can be used. The impedance level is typically determined by the surrounding circuitry, and it is usually 50 xcexa9. If the impedance of the filter and the surrounding circuitry do not match, the frequency response of the filter may change drastically. Typically in the passband frequencies the filter may attenuate too much and the frequency response may not retain its shape.
The difficulty in producing balanced filters employing BAW resonators is that when the piezoelectric layer of the BAW filters is sputter deposited on a substrate wafer, the thickness of the piezoelectric layer is not uniform throughout the wafer. The resonance frequencies of a BAW resonator are determined mainly by the acoustical properties of the piezoelectric material and the thickness of the piezoelectric layer. The resonance frequencies increase as the thickness of the piezoelectric layer decreases. The frequency response and the center frequency of a balanced filter depend on the resonance frequencies of the resonators in the filter structure. Consequently, on a wafer only a small portion of the manufactured balanced filters may thus exhibit the desired center frequency and frequency response. For example, the non-uniformity of the piezoelectric layer may cause about 8 MHz change in the center frequency of the balanced filters, when the center frequencies are of the order of 1 GHz. 8 MHz shift in the center frequency may be too much in some applications.
This variation in the center frequency causes that when manufacturing balanced filters employing BAW resonators the yield can be very low. It is possible to deposit additional layers to or remove some material from some of the resonators on a wafer. The layer structure of the BAW resonators at some parts of the wafer may be modified this way, but it is very tedious and involves extra processing steps.
This problem of a manufactured balanced filter not having a desired frequency response is acute also for balanced filters employing other resonators than BAW resonators.
There is also a problem related to the ambient temperature of the balanced filters employing BAW resonators. The resonance frequencies of a BAW resonator utilizing ZnO exhibit a temperature coefficient of, typically, xe2x88x9245 ppm/xc2x0 C. (parts per million). This means that the resonance frequencies of the BAW resonator decrease as the temperature increases. The temperature dependence is at least partly a result of the thermal expansion of the ZnO piezoelectric layer. Consequently, the frequency response of a balanced filter employing BAW resonators is temperature dependent. This may be very inconvenient especially in applications where a large operating temperature range is desirable.
The object of the invention is to present a method for adjusting the center frequency of a balanced filter. A further object is to present an adjusting method that can be applied to dynamic changes of the center frequency. Preferably the method for adjusting the center frequency of balanced filters can be applied when manufacturing balanced filters.
These and other objects of the invention are achieved by adjusting at certain limits the impedance difference between the balanced filter and the surrounding circuitry.
Method according to the invention is a method for adjusting the center frequency of a balanced filter comprising at least four resonators, and the method comprises the steps of:
specifying a nominal center frequency for the balanced filter when the balanced filter is connected to a circuitry having a certain nominal impedance,
determining the actual center frequency of the balanced filter when the balanced filter is connected to a circuitry having a certain actual impedance,
comparing the actual center frequency of the balanced filter to the nominal center frequency of the balanced filter, and
adjusting the ratio between the impedance of the circuitry and the impedance of the balanced filter within certain limits based on the comparison.
The invention relates also to a plurality of balanced filters on a certain substrate, said plurality comprising balanced filters,
each of which balanced filters has a certain number, which is at least four, of resonators connected to each other in a certain way, and which plurality of balanced filters is characterized in that
each of which balanced filters comprises at least one bulk acoustic wave resonator,
the total area of bulk acoustic wave resonators of a balanced resonator belonging to said plurality is arranged to depend on the position of the balanced filter on the substrate and
all balanced filters belonging to said plurality have substantially the same actual center frequency.
The accompanying dependent claims describe further preferred embodiments of the invention.
In a method according to the invention the center frequency of a balanced filter is changed by adjusting the ratio of the impedance of the surrounding circuitry and the impedance of the balanced filter. The frequency response of the balanced filter is designed usually so that the frequency response is centered at a certain nominal center frequency when the impedance of the balanced filter and the impedance of the surrounding circuitry have a same, predetermined value. When a slight change to the impedance ratio is caused, the center frequency shifts towards higher (when impedance ratio increases) or lower frequencies (when impedance ratio decreases) from the nominal center frequency. The shape of the frequency response stays practically unchanged.
In a method according to the invention the impedance ratio can be modified by changing the impedance of the balanced filter, the impedance of the surrounding circuitry or both impedances. If the ratio of the impedance of the surrounding circuitry and the impedance of the balanced filter is too large or too small, then the frequency response of the filter does not retain its shape. Therefore the impedance ratio can be adjusted within certain limits only.
One advantage of the invention is that it is possible to compensate dynamic changes, for example due to temperature variations, in the center frequency of a balanced filter by adjusting, for example, the termination impedance in the input and/or output ports of the balanced filter.
A further advantages of the invention is that it is possible to compensate changes in the center frequency of a balanced filter, when the changes are static and due to, for example, structural imperfections of the resonators in the balanced filter. In this case it is advantageous to modify the impedance of the balanced filter, for example the impedance of a specific resonator in the balanced filter. If the balanced filter comprises bulk acoustic wave resonators, their impedance can be adjusted by modifying the area of the resonator. If the balanced filter comprises surface acoustic wave resonators, their impedance can be adjusted by modifying the number of fingers and/or the area of the fingers. In the case of balanced filters employing BAW resonators, the modifications can be made so that, for example, a deposition pattern is partly redrawn. After the pattern has been designed and produced, the yield of balanced filters increases without adding extra step to the manufacturing process. The method according to the invention can be applied to the manufacture of balanced filters employing any kind of BAW resonators discussed in the context of prior art.
The center frequency of any balanced filter based on resonator elements and having a lattice topology shown in FIG. 7a may be tuned according to the invention by modifying the impedance of the surrounding circuitry. The compensation of unidealities in manufacturing by modifying the impedance of the resonators is probably most efficient for balanced filters employing BAW resonators. There the area of the resonators can be easily adjusted. It is also possible to modify the impedance of the filter by adding extra capacitors to the structure of a balanced filter, although this may degrade the frequency response in addition to tuning the center frequency. Adding extra coils to a balanced resonator also modifies the impedance of the filter by changing the inductance of the filter, but here the problem may be, in addition to the degradation of the frequency response, the size of the coils.